The present invention relates to a method for detecting the residual amount of a web roll, such as a carbon film roll or printing paper roll wound in roll form in a printer.
In a conventional transfer thermal printer, the unwound length of a carbon roll, for example, is automatically measured, and upon exhaustion of the roll, a warning is issued; however, it takes a substantial time to perform an operation for preparing the next carbon film roll for exchange and, as described above the user is normally given a warning, when the printer is already stopped. Further, there are cases in which there is no fresh roll at hand and the handling thereof is postponed because of the absence of time to do so, thereby rendering the printer inoperable.
The present invention is intended to provide a method of issuing a warning before carbon film roll or the like is exhausted so as to eliminate the above drawback encountered when no warning is issued unless such roll is completely expended.